1. Field of the Invention
The present invention relates to methods for accurately, noninvasively measuring the pulmonary capillary blood flow (PCBF), cardiac output, and mixed venous carbon dioxide content of the blood of a patient. Particularly, the present invention relates to a method for noninvasively measuring pulmonary capillary blood flow or cardiac output that employs an algorithm to increase the accuracy of data upon which the pulmonary capillary blood flow or cardiac output measurement is based.
2. State of the Art
Carbon dioxide elimination (VCO2) is the volume of carbon dioxide (CO2) excreted from the body of a patient during respiration. Conventionally, carbon dioxide elimination has been employed as an indicator of metabolic activity. Carbon dioxide elimination has also been used in rebreathing methods of determining pulmonary capillary blood flow and cardiac output.
The carbon dioxide Fick equation:Q=VCO2/(CvCO2−CaCO2),  (1)where Q is cardiac output, CvCO2 is carbon dioxide content of the venous blood of the patient, and CaCO2 is the carbon dioxide content of the arterial blood of the patient, has been employed to noninvasively determine the pulmonary capillary blood flow or cardiac output of a patient. The carbon dioxide elimination of the patient may be noninvasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal, or the fraction of respiratory gases that comprises carbon dioxide, or “carbon dioxide fraction,” times the rate of flow over an entire breath.
The partial pressure of end-tidal carbon dioxide (PetCO2 or etCO2) is also measured in rebreathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO2) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO2).
Rebreathing is typically employed either to noninvasively estimate the carbon dioxide content of mixed venous blood (as in total rebreathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial rebreathing). Rebreathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During rebreathing, the carbon dioxide elimination of the patient decreases to a level less than during normal breathing. Rebreathing during which the carbon dioxide elimination decreases to near zero is typically referred to as total rebreathing. Rebreathing that causes some decrease, but not a total cessation of carbon dioxide elimination, is typically referred to as partial rebreathing.
Rebreathing is typically conducted with a rebreathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates an exemplary rebreathing circuit 50 that includes a tubular airway 52 that communicates air flow to and from the lungs of a patient. Tubular airway 52 may be placed in communication with the trachea of the patient by known intubation processes, or by connection to a breathing mask positioned over the nose and/or mouth of the patient. A flow meter 72, which is typically referred to as a pneumotachometer, and a carbon dioxide sensor 74, which is typically referred to as a capnometer, are disposed between tubular airway 52 and a length of hose 60 and are exposed to any air that flows through rebreathing circuit 50. Both ends of another length of hose, which is referred to as deadspace 70, communicate with hose 60. The two ends of deadspace 70 are separated from one another by a two-way valve 68, which may be positioned to direct the flow of air through deadspace 70. Deadspace 70 may also include an expandable section 62. A Y-piece 58, disposed on hose 60 opposite flow meter 72 and carbon dioxide sensor 74, facilitates the connection of an inspiratory hose 54 and an expiratory hose 56 to rebreathing circuit 50 and the flow communication of the inspiratory hose 54 and expiratory hose 56 with hose 60. During inhalation, gas flows into inspiratory hose 54 from the atmosphere or a ventilator (not shown). During normal breathing, valve 68 is positioned to prevent inhaled and exhaled air from flowing through deadspace 70. During rebreathing, valve 68 is positioned to direct the flow of exhaled and inhaled gases through deadspace 70.
The rebreathed air, which is inhaled from deadspace 70 during rebreathing, includes air that has been exhaled by the patient (i.e., carbon dioxide-rich air).
During total rebreathing, substantially all of the gas inhaled by the patient was expired during the previous breath. Thus, during total rebreathing, the partial pressure of end-tidal carbon dioxide (PetCO2 or etCO2) is typically assumed to be equal to or closely related to the partial pressure of carbon dioxide in the arterial (PaCO2), venous (PvCO2), or alveolar (PACO2) blood of the patient. Total rebreathing processes are based on the assumption that neither pulmonary capillary blood flow nor the content of carbon dioxide in the venous blood of the patient (CvCO2) changes substantially during the rebreathing process. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve, where the change in the carbon dioxide content of the blood (CvCO2—CaCO2) is equal to the slope (s) of the carbon dioxide dissociation curve multiplied by the measured change in end-tidal carbon dioxide (PetCO2) as effected by a change in effective ventilation, such as rebreathing.
In partial rebreathing, the patient inhales a mixture of “fresh” gases and gases exhaled during the previous breath. Thus, the patient does not inhale a volume of carbon dioxide as large as the volume of carbon dioxide that would be inhaled during a total rebreathing process. Conventional partial rebreathing processes typically employ a differential form of the carbon dioxide Fick equation to determine the pulmonary capillary blood flow or cardiac output of the patient, which do not require knowledge of the carbon dioxide content of the mixed venous blood. This differential form of the carbon dioxide Fick equation considers measurements of carbon dioxide elimination, CvCO2, and the content of carbon dioxide in the alveolar blood of the patient (CACO2) during both normal breathing and the rebreathing process as follows:
                                          Q            pcbfBD                    =                                                    V                                  CO                                      2                    ⁢                    B                                                              -                              V                                  CO                                      2                    ⁢                    D                                                                                                      (                                                                            Cv                      ⁢                      CO                                                              2                      ⁢                      B                                                        -                                                            Cv                      ⁢                      CO                                                              2                      ⁢                      D                                                                      )                            -                              (                                                      CaCO                                          2                      ⁢                      B                                                        -                                      CaCO                                          2                      ⁢                      D                                                                      )                                                    ,                            (        2        )            where VCO2 B and VCO2 D are the carbon dioxide production of the patient before rebreathing and during the rebreathing process, respectively, CvCO2 B and CvCO2 D are the content of CO2 of the venous blood of the patient before rebreathing and during the rebreathing process, respectively, and CaCO2 B and CaCO2 D are the content of CO2 in the arterial blood of the patient before rebreathing and during rebreathing, respectively.
Again, with a carbon dioxide dissociation curve, the measured PetCO2 can be used to determine the change in content of carbon dioxide in the blood before and during the rebreathing process. Accordingly, the following equation can be used to determine pulmonary capillary blood flow or cardiac output when partial rebreathing is conducted:Q=ΔVCO2/sΔPetCO2.  (3)
Alternative differential Fick methods of measuring pulmonary capillary blood flow or cardiac output have also been employed. Such differential Fick methods typically include a brief change of PetCO2 and VCO2 in response to a change in effective ventilation. This brief change can be accomplished by adjusting the respiratory rate, inspiratory and/or expiratory times, or tidal volume. A brief change in effective ventilation may also be effected by adding CO2, either directly or by rebreathing. An exemplary differential Fick method that has been employed, which is disclosed in Gedeon, A. et al. in 18 Med. &Biol. Eng. &Comput. 411-418 (1980), employs a period of increased ventilation followed immediately by a period of decreased ventilation.
The carbon dioxide elimination of a patient is typically measured over the course of a breath by the following, or an equivalent, equation:VCO2=∫breathV×ƒCO2dt,  (4)where V is the measured respiratory flow and ƒCO2 is the substantially simultaneously detected carbon dioxide signal, or fraction of the respiratory gases that comprises carbon dioxide or “carbon dioxide fraction.”
Due to the measured respiratory constituents upon which VCO2 and PetCO2 calculations are made, VCO2 typically responds to rebreathing about one breath before PetCO2 for the same breath. Accordingly, a VCO2 signal may lead a PetCO2 signal by about one breath. Thus, at a particular point in time, the VCO2 and PetCO2 signals do not correspond to one another. As these values are often used to noninvasively determine pulmonary capillary blood flow or cardiac output, the lack of correspondence between these values may lead to inaccuracies in the pulmonary capillary blood flow or cardiac output determination.
In addition, measurements that are taken during spurious breaths, or breaths which do not provide information relevant to pulmonary capillary blood flow or cardiac output, may act as noise that introduces inaccuracy into the noninvasive pulmonary capillary blood flow or cardiac output determination.
When equation (4) is employed to calculate the carbon dioxide elimination of the patient from the respiratory flow and carbon dioxide fraction measurements over an entire breath, such miscorrelation or noise-induced inaccuracies in either the expiratory flow, the inspiratory flow, or both may cause inaccuracies in the carbon dioxide elimination determination or inconsistencies between carbon dioxide elimination determinations.
Accordingly, there is a need for a method of accurately, noninvasively calculating pulmonary capillary blood flow and cardiac output.